Thermosetting polymers, such as epoxy adhesives (including two-component epoxy adhesives), have been used in various industries to bond one surface of a substrate to an adjacent surface of another substrate. Two-component epoxy adhesives typically include one or more epoxy resins that are selectively combined with one or more curing agents or hardeners, such as a variety of active hydrogen compounds including polyamines, polyacids, polymercaptans, polyphenols, polyamides and polyureas. Additional materials and additives, such as extenders, fillers, reinforcing agents, colorants (e.g., pigments or dyes), organic solvents, plasticizers, flexibilizers, tackifiers, diluents, adhesion promoters, thixotropic agents, rheological agents, and the like, may be incorporated into either of the two components, as is known in the art.
With respect to the automotive industry, two-component epoxy adhesives have been used to bond stamped metal parts together, as well as SMC (i.e., sheet molded compound) to SMC, or SMC to metal (e.g., steel) parts for automotive assembly purposes. Typical automotive applications of two-component epoxy adhesives can include the bonding of SMC parts such as doors, hoods, tailgates and body panels. Accordingly, the use of these two-component epoxy adhesives has enabled automotive manufacturers to reduce the weight of vehicles, as well as realize cost and labor savings by eliminating the need for other more expensive and complicated joining methods.
Examples of epoxy adhesive compositions, including two-component epoxy adhesive compositions, can be found with reference to U.S. Pat. No. 4,383,060 to Dearlove et al.; U.S. Pat. No. 4,661,539 to Goel, U.S. Pat. No. 4,728,384 to Goel; U.S. Pat. No. 4,728,737 to Goel; U.S. Pat. No. 4,740,539 to Goel; U.S. Pat. No. 4,762,864 to Goel et al., the specifications of all of which are incorporated herein by reference. Additionally, examples of epoxy adhesive compositions, including two-component epoxy adhesive compositions, can also be found with reference to European Patent Application Nos. 0 488 949 B1 to Behm et al. and 0 527 706 A1 to Wongkamolsesh, the entire specifications of which are incorporated herein by reference.
Unfortunately, conventional two-component epoxy adhesives do suffer from several drawbacks that preclude their widespread use as an automotive adhesive, especially with respect to bonding the components of exterior automotive panels together. The main problem is that, upon curing, two-component epoxy adhesives are very rigid (i.e., inflexible) and have poor contraction and elongation characteristics. Even conventional two-component epoxy adhesives that contain flexibilizers suffer from significant problems, such as poor cross-linking density and moisture sensitivity problems. All of these issues detract from the processability and usefulness of two-component epoxy adhesives, where a relatively high degree of flexibility and low moisture sensitivity are desirable.
Additionally, the use of conventional two-component epoxy adhesives have led to read through problems in the finished exterior automotive panel, especially those using SMC, thus detracting from the aesthetic appearance of the finished exterior automotive panel. This problem causes high defects rates in the finished exterior automotive panels and increases manufacturing costs.
Further, adhesive failure has occurred with conventional two-component epoxy adhesives, especially after exposure to elevated temperature, such as during hot water immersion. Accordingly, automotive manufacturers have resorted to using more expensive acrylic-based adhesives, especially for assembling and bonding exterior panels.
Therefore, there exists a need for effective and inexpensive two-component epoxy adhesive compositions that have improved flexibility and elongation characteristics and that are especially useful for bonding together the components of exterior automotive panels.